st: merge by nearest date

I am using Stata ver 8.2 and wish to merge two files each with an idcode
and a date. (I know that more recent versions of Stata can handle this
more capably but funds are short at this time so I make do with what I
have.) None of the dates in using matches exactly with master. I get
the following:
. tab _merge
_merge | Freq. Percent Cum.
------------+-----------------------------------
1 | 585 1.82 1.82
2 | 31,554 98.18 100.00
------------+-----------------------------------
Total | 32,139 100.00
This is what I expected but I would like to see how close I can get a
pair (master and using) that are within one week of one another. If I
calculate a simple difference between master and the next using above
will miss any closer using if it is below the master. I hope this makes
sense. Here is a snippet of the data:
idcode visit _merge
FEL00057 26 Sep 33 2
FEL00057 30 Sep 33 1
FEL00057 22 Nov 33 2
FEL00057 24 Jan 34 2
FEL00057 28 May 34 2
FEL00057 03 Aug 34 2
FEL00057 23 Sep 34 1
FEL00057 24 Sep 34 2
FEL00057 27 Nov 34 2
FEL00057 28 Jan 35 2
FEL00057 27 Mar 35 1
FEL00057 30 Mar 35 2
If my data is arranged in this fashion, how do I best get the closest
pair of using and master?
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